Annexins are a family of calcium- and phospholipid-binding proteins whose proposed functions include membrane fusion, ion channel formation, and cell-cycle regulation. The crystal structure of the annexin XII hexamer from Hydra vulgaris was solved at 2.5 w resolution by molecular replacement, using the human annexin I monomer (44% sequence identity) as the model. The hexamer forms a concave disk with 32 symmetry, about 100 w in diameter and 70 w thick with a central hydrophilic channel, burying 19% of the solvent-accessible surface of each monomer. The hexamer is further stabilized by six calcium ions, liganded by atoms from2the two opposing trimers, and six lysine e-amino groups replacing Ca in the domain II calcium-binding loop. The hexameric structure has led us to propose a new model of annexin-membrane interaction (H. Luecke et al., Nature 378, 512-515 (1995)). Small crystals (0.05 mm x 0.15 mm x 0.15 mm) of annexin XII were grown at SSRL, necessitating X-ray diffraction data collection at a synchrotron source. A partially complete native data set of a frozen crystal diffracting to 2.5 w resolution was collected at beam line 7-1 at SSRL.